An LED-array-based range imaging system is proposed for three-dimensional (3-D) shape measurement. The range image
is obtained by time-division electronic scanning of the LED Time-of-Flight (TOF) range finders in array, and no
complex mechanical scanning is needed. By combining with a low cost CCD/CMOS sensor for capturing the twodimensional
(2-D) image, the proposed range imaging system can be used to accomplish a high quality 3-D imaging. A
sophisticated co-lens optical path is designed to assure the natural registration between the range image and 2-D image.
Experimental tests for evaluation of the imaging system performance are described. It was found that the 3-D images can
be acquired at a rate of 10 frames per second with a depth resolution better than 5mm in the range of 50 - 1000mm,
which is sufficient for many practical applications, including the obstacle detection in robotics, machine automation, 3-D
vision, virtual reality games and 3-D video.
A high-resolution, real-time range imaging system based on a LED array phase-shift range finder was developed to measure three-dimensional surface profiles. The range image is captured by a time-division scanning LED array heterodyne phase-shift range finder. The phase measurement used for obtaining depth is implemented by using the fast "four-bucket" algorithm. The design of a fast automatic gain control circuit obviously reduces the feedback response time and improves system measurement speed. Experimental tests for evaluation of the imaging system performance are described. Based on the theoretical analysis, a simplified signal-to-noise ratio model is proposed for guiding the design of this imaging system. Last, some issues requiring attention on constructing a large-pixel version of the system are also discussed. It was found that the range images can be acquired at a rate of 10 frames per second with a depth resolution better than ±5 mm in the range of 50 to 1000 mm.
KEYWORDS: Modulation, Vertical cavity surface emitting lasers, Signal processing, Ranging, Laser range finders, Phase shift keying, Digital signal processing, Analog electronics, Distortion, Complex systems
The paper proposes a small modulation and high precision laser range finder using the self-mixing effect in a
single-mode vertical-cavity surface-emitting laser (VCSEL). According to the characteristic curve of single-mode
VCSEL, the nonlinear distortion of Δλ/ΔI is severe and related with the modulation current ΔI, which worsens the system
ranging accuracy obviously. In this paper, by applying small current to modulate single-mode VCSEL and specific
circuits to process photoelectric signal, the system ranging accuracy is improved obviously. The experiment results show
that, using small modulation current ΔI= 0.28 mAp-p, modulation frequency fm = 500 Hz to modulate single-mode
VCSEL, and applying difference frequency analog phase-locked loop (APLL) to process the self-mixing beat frequency
signals that exist phase abrupt changes, when the sampling time is 0.1 s, the measurement dynamic range is as large as 50
~ 500 mm, and the ranging accuracy is better than 2 mm.
The experiments show that the single-mode vertical-cavity surface-emitting laser (VCSEL) Laser Doppler Velocimetry (LDV) is better than the multilongitudinal-mode laser diode LDV in many characteristics, such as the accuracy of velocity measurement and the temperature range of the laser. Because the output power of the single-mode VCSEL is very low and only 0.7mW, the backscattered light received by the photodiode in the laser house is so weak that the signal-to-noise ratio (SNR) of Doppler signal is low. And the Doppler signal spectrum width spread and amplitude modulation envelope badly influence the velocity measurement accuracy. Analog phase-locked loop (PLL) has many characteristics, such as narrow band tracking filter, locking the signal with peak voltage and high rejection ratio of amplitude modulation. Using the analog PLL, the SNR of Doppler signal and the velocity measurement accuracy can be improved obviously. But because the locking range of the common analog PLL is narrow, we use difference frequency analog PLL to expand the locking range, and the dynamic range of velocity measurement can be greatly expanded. As a result, the velocity measurement accuracy of the single-mode VCSEL self-mixing LDV is better than 1% when the velocity range is from 30mm/s to 480mm/s.
A new, compact laser Doppler velocimetry is proposed, which is composed of a single-mode vertical-cavity surface-emitting laser modulated by a dynamical triangular current and a collimating lens. It can indicate the direction of velocity without ambiguity in the wide dynamic range of 5.2mm/s to 479.9mm/s when the sampling time is 0.1 second in the measurement. The accuracy of velocity measurement is better than 3.1% in the whole velocity range. What's more, this LDV works very well on different diffusing surfaces, even on a black glossy photographic paper.
This paper presents a theoretical and numerical analysis of polarization properties used as Doppler velocimetry in Vertical-Cavity Surface-Emitting lasers. A good match is found between numerical results and reported experiment results. We show that condition of generating square waves is the VCSELs must operated inside the bistable region and frequency difference between two linear polarization modes is equal to several certain values. We can select proper VCSELs which frequency difference between two linear polarization modes is multiple of ±3G to obtain asymmetric waveform of self-mixing signal.
SM-LDV (Self-Mixing Type Laser Doppler Velocimeter) using single mode VCSEL (Vertical Cavity Surface Emitting Lasers) has many characteristic such as high accuracy of velocity measurement, low power dissipation and low cost. The saw-tooth waveform of Doppler signal can be used to discriminate the motion direction. Because of low light power of single mode VCSEL, the backscattered light is so weak that the saw-tooth waveform is not good enough to discriminate the motion direction. SM-LDV direction discrimination using single mode VCSEL modulated by triangular wave is not obviously dependent on the light power of Laser and its reliability is better. The dynamic range is very narrow when the VCSEL is modulated by constant frequency triangular wave. We present a method that VCSEL is modulated by triangular wave whose frequency is tracking the divided frequency of Doppler frequency shift (fD) and it can expand the dynamic range more than one order of magnitude. The results show that the dynamic range of direction discrimination can reach 5-500mm/s when the wavelength of single mode VCSEL is 850nm and the operating current is 8.6mA, the frequency of triangular wave is fD/16,the modulation current of triangular wave is 0.042mA.
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